Network Working Group S. Bellovin
Request for Comments: 1948 AT&T Research
Category: Informational May 1996

Defending Against Sequence Number Attacks

Status of This Memo

This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.

Abstract

IP spoofing attacks based on sequence number spoofing have become a
serious threat on the Internet (CERT Advisory CA-95:01). While
ubiquitous crypgraphic authentication is the right answer, we propose
a simple modification to TCP implementations that should be a very
substantial block to the current wave of attacks.

Overview and Rational

In 1985, Morris [1] described a form of attack based on guessing what
sequence numbers TCP [2] will use for new connections. Briefly, the
attacker gags a host trusted by the target, impersonates the IP
address of the trusted host when talking to the target, and completes
the 3-way handshake based on its guess at the next initial sequence
number to be used. An ordinary connection to the target is used to
gather sequence number state information. This entire sequence,
coupled with address-based authentication, allows the attacker to
execute commands on the target host.

Clearly, the proper solution is cryptographic authentication [3,4].
But it will quite a long time before that is deployed. It has
therefore been necessary for many sites to restrict use of protocols
that rely on address-based authentication, such as rlogin and rsh.
Unfortunately, the prevalence of "sniffer attacks" -- network
eavesdropping (CERT Advisory CA-94:01) -- has rendered ordinary
TELNET [5] very dangerous as well. The Internet is thus left without
a safe, secure mechanism for remote login.

We propose a simple change to TCP implementations that will block
most sequence number guessing attacks. More precisely, such attacks
will remain possible if and only if the Bad Guy already has the
ability to launch even more devastating attacks.

In order to understand the particular case of sequence number
guessing, one must look at the 3-way handshake used in the TCP open
sequence [2]. Suppose client machine A wants to talk to rsh server
B. It sends the following message:

A->B: SYN, ISNa

That is, it sends a packet with the SYN ("synchronize sequence
number") bit set and an initial sequence number ISNa.

B replies with

B->A: SYN, ISNb, ACK(ISNa)

In addition to sending its own initial sequence number, it
acknowledges A's. Note that the actual numeric value ISNa must
appear in the message.

A concludes the handshake by sending

A->B: ACK(ISNb)

The initial sequence numbers are intended to be more or less random.
More precisely, RFC 793 specifies that the 32-bit counter be
incremented by 1 in the low-order position about every 4
microseconds. Instead, Berkeley-derived kernels increment it by a
constant every second, and by another constant for each new
connection. Thus, if you open a connection to a machine, you know to
a very high degree of confidence what sequence number it will use for
its next connection. And therein lies the attack.

The attacker X first opens a real connection to its target B -- say,
to the mail port or the TCP echo port. This gives ISNb. It then
impersonates A and sends

goes to the legitimate A, about which more anon. X never sees that
message but can still send

Ax->B: ACK(ISNb')

using the predicted value for ISNb'. If the guess is right -- and
usually it will be -- B's rsh server thinks it has a legitimate
connection with A, when in fact X is sending the packets. X can't
see the output from this session, but it can execute commands as more
or less any user -- and in that case, the game is over and X has won.

There is a minor difficulty here. If A sees B's message, it will
realize that B is acknowledging something it never sent, and will
send a RST packet in response to tear down the connection. There are
a variety of ways to prevent this; the easiest is to wait until the
real A is down (possibly as a result of enemy action, of course). In
actual practice, X can gag A by exploiting a very common
implementation bug; this is described below.

The Fix

The choice of initial sequence numbers for a connection is not
random. Rather, it must be chosen so as to minimize the probability
of old stale packets being accepted by new incarnations of the same
connection [6, Appendix A]. Furthermore, implementations of TCP
derived from 4.2BSD contain special code to deal with such
reincarnations when the server end of the original connection is
still in TIMEWAIT state [7, pp. 945]. Accordingly, simple
randomization, as suggested in [8], will not work well.

But duplicate packets, and hence the restrictions on the initial
sequence number for reincarnations, are peculiar to individual
connections. That is, there is no connection, syntactic or semantic,
between the sequence numbers used for two different connections. We
can prevent sequence number guessing attacks by giving each
connection -- that is, each 4-tuple of <localhost, localport,
remotehost, remoteport> -- a separate sequence number space. Within
each space, the initial sequence number is incremented according to
[2]; however, there is no obvious relationship between the numbering
in different spaces.

The obvious way to do this is to maintain state for dead connections,
and the easiest way to do that is to change the TCP state transition
diagram so that both ends of all connections go to TIMEWAIT state.
That would work, but it's inelegant and consumes storage space.
Instead, we use the current 4 microsecond timer M and set

It is vital that F not be computable from the outside, or an attacker
could still guess at sequence numbers from the initial sequence
number used for some other connection. We therefore suggest that F
be a cryptographic hash function of the connection-id and some secret
data. MD5 [9] is a good choice, since the code is widely available.
The secret data can either be a true random number [10], or it can be
the combination of some per-host secret and the boot time of the
machine. The boot time is included to ensure that the secret is
changed on occasion. Other data, such as the host's IP address and
name, may be included in the hash as well; this eases administration
by permitting a network of workstations to share the same secret data
while still giving them separate sequence number spaces. Our
recommendation, in fact, is to use all three of these items: as
random a number as the hardware can generate, an administratively-
installed pass phrase, and the machine's IP address. This allows for
local choice on how secure the secret is.

Note that the secret cannot easily be changed on a live machine.
Doing so would change the initial sequence numbers used for
reincarnated connections; to maintain safety, either dead connection
state must be kept or a quiet time observed for two maximum segment
lifetimes after such a change.

A Common TCP Bug

As mentioned earlier, attackers using sequence number guessing have
to "gag" the trusted machine first. While a number of strategies are
possible, most of the attacks detected thus far rely on an
implementation bug.

When SYN packets are received for a connection, the receiving system
creates a new TCB in SYN-RCVD state. To avoid overconsumption of
resources, 4.2BSD-derived systems permit only a limited number of
TCBs in this state per connection. Once this limit is reached,
future SYN packets for new connections are discarded; it is assumed
that the client will retransmit them as needed.

When a packet is received, the first thing that must be done is a
search for the TCB for that connection. If no TCB is found, the
kernel searches for a "wild card" TCB used by servers to accept
connections from all clients. Unfortunately, in many kernels this
code is invoked for any incoming packets, not just for initial SYN
packets. If the SYN-RCVD queue is full for the wildcard TCB, any new
packets specifying just that host and port number will be discarded,
even if they aren't SYN packets.

To gag a host, then, the attacker sends a few dozen SYN packets to
the rlogin port from different port numbers on some non-existent
machine. This fills up the SYN-RCVD queue, while the SYN+ACK packets
go off to the bit bucket. The attack on the target machine then
appears to come from the rlogin port on the trusted machine. The
replies -- the SYN+ACKs from the target -- will be perceived as
packets belonging to a full queue, and will be dropped silently.
This could be avoided if the full queue code checked for the ACK bit,
which cannot legally be on for legitimate open requests. If it is
on, RST should be sent in reply.

Security Considerations

Good sequence numbers are not a replacement for cryptographic
authentication. At best, they're a palliative measure.

An eavesdropper who can observe the initial messages for a connection
can determine its sequence number state, and may still be able to
launch sequence number guessing attacks by impersonating that
connection. However, such an eavesdropper can also hijack existing
connections [11], so the incremental threat isn't that high. Still,
since the offset between a fake connection and a given real
connection will be more or less constant for the lifetime of the
secret, it is important to ensure that attackers can never capture
such packets. Typical attacks that could disclose them include both
eavesdropping and the variety of routing attacks discussed in [8].

If random numbers are used as the sole source of the secret, they
MUST be chosen in accordance with the recommendations given in [10].

Acknowledgments

Matt Blaze and Jim Ellis contributed some crucial ideas to this RFC.
Frank Kastenholz contributed constructive comments to this memo.